The simple a-and g-pyrones and the substituted 6-methyl, 4-hydroxy and 4-methoxy a-pyrones were investigated in relation to their spectroscopic properties. The characterization involves vibrational and electronic spectroscopy. IR spectra for the ground electronic state of the studied compounds at room temperature were obtained and interpreted taking into consideration the simulated ab initio (6-31G p ) data. The most important canonical structures accounting for the properties of the ground state were determined for each compound and several photophysical properties were evaluated: electronic spectra, oscillator strengths and the emission properties (both¯uorescence and phosphorescence) in media of different polarity. The origin of the ®rst singlet and triplet excited states is also discussed. In addition, the energies of the electronic transitions were estimated using semi-empirical molecular orbital calculations, the calculated results showing an excellent agreement with the experimental data. q

The molecular structures and vibrational spectra of the monomeric forms of a-and g-pyrones were investigated by ab initio MO calculations, undertaken at the HF/6-31G p level of theory, and vibrational spectroscopy, including matrix-isolation FTIR spectroscopy. A complete assignment of the vibrational spectra of the studied compounds isolated in an argon matrix, at 8 K, or in the condensed phases, at room temperature, is presented and the vibrational data correlated with some important structural parameters. It is shown that the intermolecular interactions in the room temperature condensed phases do not affect the structure and vibrational properties of the studied molecules strongly, though the vibrational results clearly reveal minor changes induced in the carbonyl groups that indirectly affect the electron distribution in the whole pyrone rings, leading to an increase in their pelectron delocalization. For the isolated monomers, both structural and vibrational results point to a relatively weak p-electron delocalization in both a-and g-pyrone moieties. q